Allyl radical formation

This reaction leads to selective formation of the 1-heptyl radical (Demeyer et al. 1993). In the case of trans-octene-2, the cation-radical deprotonation predominantly leads to the allyl radical formation (Fel dman et al. 1993, 1996). 

The hypothesis of a bifunctional mechanism involving allyl radical formation and oxygen incorporation on distinct sites is advocated by Haber et al. [147,152], This hypothesis is particularly based on experiments with Mo03, Bi203 and mechanical mixtures of these oxides, which are compared with bismuth molybdate catalysts. The reaction was carried out in cyclic operation (alternating feeds of oxygen and of propene diluted with nitrogen). The results are collected in Table 5. The authors con-... 

Experiments by Haber (79) suggest that the reaction on bismuth molybdates proceeds via allyl radical formation on oxygens associated with bismuth, since dimerization to 1,5-hexadiene and benzene occurs on Bi2O3 (Eq. 16), while oxygens of molybdenum polyhedra are oxygen inserters, since acrolein is formed from the reaction of MOO3 with allyl radical preformed from allyl iodide (Eq. 17) ... 

The results show that the rearrangements do not proceed via a single pathway. They suggest that in the case of 30a 60% migrates concertedly with allyl inversion, the remaining 40% reacting by a multistep mechanism, such as allyl radical formation. 

The diphenyl and phenyl-1-naphthyl systems react solely via path B, since the concerted attack of the allyl group at the backside of the carbon atom is sterically inhibited. This induces 100% allyl radical formation, in line with the observed deuterium scrambling in the allyl group. Such a process represents a unique case of anchimerically accelerated unimolecular bond homolysis and will be treated in detail in Section IV. 

The resulting crosslinking measured as swelling ratio in cyclohexane (25°C) show clearly that ENB is the most effective diene and HD the least effective (Fig. 8). The low rate of reaction for the HD copolymer is interpreted as due to addition of initiator radicals which in this case is competing with allyl radical formation. 

We and others countered these arguments by invoking effects of mutual displacement of exchange partners in the chemisorption layer (75,76), the likelihood of the allyl radical formation from propene (77) and the proof for the occurrence of exchange reaction between deuturated ethylene and butene-1 (78). 

Clearly propylene and ethylene are Important during pyrolysis In the production of coke. The high reactivity of ethylene as compared to propylene Is thought to be caused primarily by the lower stability of vinyl radicals, as compared to that of allyl radicals. Formation of the Cg - Cg Intermediates possibly occurs by dimerization and/or trlmerization of these radicals. 

The allylic resonance may give rise to formation of a mixture of isomeric allylic bromides, e.g. 6 and 8 from but-l-ene. The product ratio depends on the relative stability of the two possible allylic radical species 5 and 7 ... 

As mentioned in an earlier section (cf. Chapter 1, Section III), allylic positions are subject to attack by free radicals resulting in the formation of stable allyl radicals. A-Bromosuccinimide (NBS) in the presence of free-radical initiators liberates bromine radicals and initiates a chain reaction bromination sequence by the abstraction of allylic or benzylic hydrogens. Since NBS is also conveniently handled, and since it is unreactive toward a variety of other functional groups, it is usually the reagent of choice for allylic or benzylic brominations (7). 

Several studies characterizing the reactions of alkenyl radicals with quinone dumines and quino-neimines were published in the late 1970s. Quinone dumines react with allylic radicals yielding both the reduced PPD and the alkylated product. In these experiments 2-methyl-2-pentene served as a model olefin (model for NR). Samples of the olefin and quinoneimines or quinone diimine were heated to 140°C. Isolation and analysis of products demonstrated that 40%-70% of the imine or diimine was reduced to the corresponding PPD, while 20%-50% was isolated as the alkylated product. This alkylation reaction (via an allylic radical) represents the pathway to the formation of rubber-bound antidegradant. ... 

Matrix IR spectra of various silenes are important analytical features and allow detection of these intermediates in very complex reaction mixtures. Thus, the vibrational frequencies of Me2Si=CH2 were used in the study of the pyrolysis mechanism of allyltrimethylsilane [120] (Mal tsev et al., 1983). It was found that two pathways occur simultaneously for this reaction (Scheme 6). On the one hand, thermal destruction of the silane [120] results in formation of propylene and silene [117] (retroene reaction) on the other hand, homolytic cleavage of the Si—C bond leads to the generation of free allyl and trimethylsilyl radicals. While both the silene [117] and allyl radical [115] were stabilized and detected in the argon matrix, the radical SiMc3 was unstable under the pyrolysis conditions and decomposed to form low-molecular products. 

Isopropenyl acetate and allyl chloride behave similarly. In the polymerization of the latter monomer degradative chain transfer occurs more readily by removal of the chlorine atom to yield the unsubstituted allyl radical CH2—CH—CH2, which manages to add monomer occasionally. This is indicated by the formation of about three polymer molecules, having an average degree of polymerization of six units, for each molecule of benzoyl peroxide decomposing. 

The observation that in the case of PCSO there is no formation of propanol while allyl alcohol is formed from ACSO agrees with the resonance stabilization of the allyl radical and hence weaker bond for S-allyl than for S-propyl. The yield of allyl alcohol from irradiation of ACSO is considerably greater than that from S-allyl-L-cysteine, probably due to energy delocalization by the four p electrons of the S atom. 

It has been reported that molecnlar oxygen plays an important role in the allylic oxidation of olefins with TBHP (25, 26). Rothenberg and coworkers (25) proposed the formation of an alcoxy radical via one-electron transfer to hydroperoxide, Equation 4, as the initiation step of the allylic oxidation of cyclohexene in the presence of molecnlar oxygen. Then, the alcoxy radical abstracts an allylic hydrogen from the cyclohexene molecnle. Equation 5. The allylic radical (8) formed reacts with molecular oxygen to yield 2-cyclohexenyl hydroperoxide... 

Fig. 20. Possible dissociation channels of allyl radical and their standard heats of formation relative to allyl. The loss of H2 generally proceeds via a high activation barrier and is thus considered unlikely. (From Fischer et ai.14B)...

With propene, CH3CH=CH2 (79), there is the possibility of either addition of chlorine to the double bond, or of attack on the CH3 group. It is found that at elevated temperatures, e.g. 450° (Cl then being provided by thermolysis of Cl2), substitution occurs to the total exclusion of addition. This is because the allyl radical (80) obtained by H-abstraction is stabilised by delocalisation, whereas the one (81) obtained on Cl addition is not, and its formation is in any case reversible at elevated temperatures, the equilibrium lying over to the left ... 

As in the case of linear peroxidation products, the initiation step of the formation of isoprostanes is the abstraction of a hydrogen atom from unsaturated acids by a radical of initiator. Initiation is followed by the addition of oxygen to allylic radicals and the cyclization of peroxyl radicals into bicyclic endoperoxide radicals, which form hydroperoxides reacting with hydrogen donors. 

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